This invention relates generally to combustors and, more particularly, to means effective for reducing thermally induced stresses therein.
Increased performance levels of gas turbine engines can be obtained by increasing the operating temperatures thereof. In so doing, the combustors of these gas turbine engines are exposed to extremely high temperatures and large variations in temperatures during engine operation. In order to withstand these relatively high and fluctuating temperatures, new alloys have been developed which have improved material characteristics.
Furthermore, improved combustor cooling structures and techniques have been developed for reducing the temperatures experienced by the combustor. For example, one type of combustor includes a plurality of annular liner segments which define an outer boundary for constraining relatively hot combustion gases. These liner segments are joined by annular cooling nuggets, or junctions, which receive relatively cool, compressor discharge air and direct it in a cooling film along inner surfaces of the liner segments. The cooling film is effective for reducing the temperatures experienced by the liner segments.
However, during operation combustors typically experience variations in temperature throughout the different structural portions thereof. For example, the liner segments, which are heated by the combustion gases, experience higher temperatures than the cooling nuggets, which are cooled by the compressor discharge air. Inasmuch as combustor materials expand when heated, variations in temperature of these combustor portions result in differential thermal expansion of these portions, or an attempt to expand differentially where restrained from doing so, which induces thermal stresses and distortions.
More particularly, the hot liner segments tend to expand more than the cooling nuggets. However, the liner segments are attached to the nuggets and both oppose this differential thermal growth, resulting in thermally induced stress. The magnitude of the thermal stress and the number of cycles thereof is reflected in the thermal fatigue life of the material. The more cycles and the higher the thermal stress, the sooner a combustor component must be replaced prior to reaching the limit of its thermal fatigue life.
Furthermore, thermally induced distortions can be significant where they cause a typical overhanging lip adjacent to conventional cooling nuggets to grow radially outwardly and partially close off film cooling slots in the combustor. In this situation, means are typically provided for reducing flow restriction of the cooling slot.
Examples of conventional combustors including cooling nuggets and overhanging lips are disclosed in U.S. Pat. No. 4,259,842-Koshoffer et al, U.S. Pat. No. 4,050,241-DuBell, U.S. Pat. No. 3,995,422-Stamm, and U.S. Pat. No. 3,845,620-Kenworthy, all assigned to the assignee of the present invention and incorporated herein by reference. These patents also disclose the combustor environment, including hot gases and cooling air, and a few arrangements for reducing cooling slot flow restriction caused by differential thermal growth which may exist. However, in all these combustor arrangements, differential thermal expansion still produces significant thermal stress.
Accordingly, it is an object of the present invention to provide a new and improved combustor liner.
Another object of the present invention is to provide a new and improved combustor liner that is effective for reducing thermal stress due to differential thermal expansion.
Another object of the present invention is to provide a new and improved combustor liner that is effective for reducing thermal distortion due to differential thermal expansion.